The present invention relates to a variable speed reproducing apparatus for a digital video cassette recorder, and more particularly, to an apparatus which prevents data detection errors and improves synchronization performance, by removing outputs having an amplitude level less than or equal to a predetermined level from reproduction signals.
In a conventional digital videocassette recorder in which reproduction is performed at variable speeds, since either a simple AGC amplifier is used or a reproduced output is used without amplification, a portion of the reproduction signal sent to a data detector has a low carrier-to-noise (C/N) ratio and accordingly causes errors to be generated. The resultant unstable operation of a phase-locked loop (PLL) increases the bit error rate. Also, since the low C/N ratio portion is included in the output of an equalizer, error generation is further increased so that unreliable data is output therefrom, which destabilizes PLL operation at the time of data detection. As a result, the bit error rate of variable speed reproduction is high, thereby lowering system performance.
That is, when reproducing in a digital videocassette recorder as above, since the amplitude of the reproduction signal is below a predetermined level, portions of the amplified reproduction signal have a poor C/N ratio and coexist with portions having higher C/N ratios. The low-amplitude portion raises the probability of error generation at the time of data detection and lowers the synchronization performance of the PLL circuit.
A conventional variable speed magnetic recording and reproducing apparatus is disclosed in U.S. Pat. No. 4,930,024. This apparatus includes a variable speed rotary head, a variable speed disk drive and a circuit for controlling the speed of the rotary head according to the change of speed of the variable speed disk drive, and can lower noise generated during reproducing. However, according to this apparatus, a selective output according to a level of the reproduced output cannot be accomplished.